This invention relates generally to the measurement of power, and in particular to the measurement of power present in a radio frequency signal over a broad range of frequencies and signal levels.
It is frequently desirable in working with radio frequency apparatus to have the capability to monitor the amount of power present in a radio frequency signal. The amount of power present in a radio frequency signal can be used to determine the efficiency of associated apparatus. In particular, by monitoring the amount of power present in a radio frequency signal in response to adjustments to associated apparatus, the effects of such adjustments can be immediately determined. Such an approach to the adjustment process could be applied not only to a radio frequency transmitter, but also to apparatus associated therewith, e.g., matching devices, transmission lines and antennas.
The monitoring of power present in a radio frequency signal can also be useful in the development of antennas. In particular, antennas are frequently designed to direct the radiation of a radio frequency signal in a selected direction. Consequently, apparatus for determining the power present in a radio frequency signal would be useful in empirically determining the pattern of radiation from an antenna.
It is desirable to have a method for measuring the power level of a radio frequency signal through a wide dynamic range of power levels. Such a power measuring device would minimize the use of external attenuators which quickly become cumbersome to use during measurements.
In addition to the foregoing, radio transmitters frequently are capable of producing radio frequency signals over a broad range of frequencies. Consequently, it is desirable that power monitoring apparatus have a similar capability to measure the amount of power in a radio frequency signal over a corresponding broad range of frequencies.
Various techniques are employed in measuring the amount of power present in a radio frequency signal. One such technique employs the use of a thermocouple device, wherein a radio frequency signal is applied to a load physically coupled to the thermocouple, and the resulting heat generated is measured. As the amount of heat produced is proportional to the power delivered to the load, the amount of power associated with the radio frequency signal may be correspondingly determined. While such an approach does give an indication of the amount of power present in a radio frequency signal, such an approach does have a number of disadvantages. In particular, such measurements are accurate over only a limited range of signal levels due to limitations of the thermocouple.
An alternate approach to the measurement of radio frequency power has employed the use of diodes. In particular, as the relationship between voltage and current in a diode is fundamentally a square law relationship at low signal levels, diodes may be used in determining the amount of power present in a radio frequency signal. However, as the square law relation between voltage and current in a diode is only valid over a portion of the overall voltage current relationship of a diode, such an approach does have limitations. Use of such an approach is necessarily limited to signal levels which are within the limited square law region of the diode voltage current relationship. Consequently, such apparatus is limited in the range of signal levels with which it may be used. Other apparatus frequently used to measure the amount of power present in a radio frequency signal include multipliers and directional couplers. However, such apparatus is not only expensive, but often limited in the dynamic range of signals which it is capable of measuring.